1. Field of the Invention
This invention relates to a system and method for extracting contaminants from stormwater runoff.
2. Description of the Related Art
Highway stormwater runoff enters coastal area waterways and is a major contributor to the degradation of marine environments. The U.S. Environmental Protection Agency (USEPA) is now mandating coastal States and communities to develop regulations consistent with the Federal Coastal Zone Reauthorization Act (CZRA) to curb the degradation of coastal waters caused by stormwater pollution. The USEPA is phasing in new Water Quality Standards that will require states to impose Total Maximum Daily Levels (TMDL""s) on the dissolved portion of toxic metals. According to the National Transportation Research Board, heavy metals, i.e copper, lead and zinc, are typically found in most highway runoff areas. Metals speciation is very complex and requires a complete understanding of the adsorption and dissolution qualities of the metals typically found in highway runoff. Metals are known to adsorb to the very fine particulates, i.e silt size or smaller, and are not typically captured by traditional or the second-generation oil/sediment separators. Approximately 60 to 70% of the metals typically found in highway runoff are in the particulate form. However, these metals adsorb to the very fine particulates, i.e particulates less than 60 microns. Most existing second generation oil/sediment separators, such as non-vegetative BMP""s, are not designed to remove either silt size particulates or the dissolved portion of metals. At best, the existing prior second generation oil/sediment separators treat about 25% of the total concentration of metals. The present invention addresses this problem by providing a cost-effective system that removes not only the large particulates but also the dissolved fractions of toxic material associated with highway runoff.
Broadly, the invention comprises a stormwater treatment system that efficiently removes contaminants from stormwater. The system comprises a housing having a first aperture positioned upstream of a second aperture. A first filter layer and a second filter layer are positioned within the housing and a separator layer is disposed between the first and second filter layers. Stormwater flows through the first aperture and permeates through the first layer. The first layer absorbs contaminants from the stormwater to produce a first treated stormwater having an effluent concentration that meets the USEPA Water Quality Criteria The first treated stormwater flows through the separator layer, into the second filter layer and permeates through the second filter layer. The pH of the second filter layer is controlled to precipitate substantially all of the dissolved contaminants remaining in the first treated stormwater to produce a second treated stormwater. The second treated stormwater flows out of the housing through the second aperture. The second aperture can be connected to an outlet pipe.
In a preferred embodiment of the invention, the fist filter layer is comprised of granular activated carbon and the second layer comprises crushed concrete, the main component of which is calcium oxide (CaO). The separator layer is a porous membrane, preferably a geosynthetic filter fabric. The separator layer separates the first and second filter layers in order to prevent the mixing of the materials which comprise the filter layers. Further, the separator layer provides support for the first filter layer. Examples of contaminants that the first and second layers remove from the stormwater include phosphorous, cadmium, chromium, copper, lead and zinc.
In another embodiment of the invention the second filter layer comprises crushed concrete, a building material well known to one skilled in the art, e.g. Portland Concrete, having an effective amount of CaO therein to control the pH of the second filter layer within a range of between about 9-11, preferably 10, thereby precipitating substantially all of the dissolved contaminants remaining in the first treated stormwater.
The invention also comprises a method for treating stormwater which comprises permeating stormwater runoff through a first medium to produce a first treated stormwater having an effluent concentration that meets the USEPA Water Quality Criteria and flowing the first treated stormwater through a second medium having a pH within a range of between about 9-11, preferably 10, to precipitate substantially all of the dissolved contaminants remaining in the first treated stormwater.
In a preferred embodiment of the method, the first medium is comprised of granulated activated carbon and the second medium comprises crushed concrete, the main component of which is CaO. The method further comprises flowing the first treated stormwater through a separator layer. The separator layer can be comprised of a porous membrane.